Systems that produce a radiographic image by passing a beam of penetrative radiation through a subject and impinging the radiation beam on an image-producing element, such as a film disposed in a scintillating film cassette, generally employ one or more radiation absorbing grid assemblies interposed between the subject and the element. The grid assemblies reduce clouding of the film image caused by scattered radiation. If, however, the grid assemblies are stationary during radiation exposure the radiation absorbing characteristics of the grid cause an image of the grid strips, i.e. grid lines, to form on the film.
Various approaches have been devised to minimize the appearance of grid lines. One approach was to sinusoidally oscillate the grid between two endpoints such that the grid was moving during an appreciable period of the exposure to blur the grid lines. Another approach is disclosed in U.S. Pat. No. 2,767,323 to Stava et al. which suggests a motor and cam combination to oscillate the grid assembly such that the grid velocity is relatively constant during a majority of the grid oscillation and accelerates quickly through an endpoint thereby minimizing the time the grid remains stationary at the endpoint. Thus in the Stava patent the motion of the grid assembly over time is not sinusoidal but more of a triangular shaped waveform. Still another approach to minimize grid lines is disclosed in U.S. Pat. No. 4,380,086 to Vagi which suggests altering the displacement of the grid assembly motion endpoints during successive oscillations such that the positions of the stationary point of the grid motion at an endpoint is not repeated during a single exposure.
All of these approaches achieve a reduction in the appearance of grid lines for exposures of approximately 0.5 seconds or longer in duration. However, as exposure times become progressively shorter, for example 0.05 seconds, the independent functions of grid motion and exposure time occasionally coincide such that the grid is at or near zero velocity at an endpoint during exposure thereby resulting in random grid lines appearing on the film. The Stava Patent, referenced above, suggests overcoming this problem by fixing the exposure start to grid motion during an exposure sequence. A motor/cam arrangement is proposed which, after an exposure sequence is initiated, commences grid motion. A fixed period of time later, the exposure is initiated. In this fashion, the exposure occurs during the substantially constant velocity portion of the grid motion. One drawback to the Stava invention is that the cams and switches must be mechanically adjusted to change the exposure duration or the relationship of the exposure start to the grid motion. Another drawback is that the Stava invention cannot be dynamically adjusted to account for imaging system events that occur prior to the actual exposure, such as the x-ray tube rotor start-up and the x-ray tube filament preheat. Still another drawback is that for exposure durations greater than the time required for one revolution of the cam, the exposure start switch must be manually maintained in the closed position to complete the exposure.
As radiographic imaging technology has progressed, one feature that evolved is the ability to take a plurality of relatively short duration rapidly sequenced exposures on different portions of a single sheet of film by sequentially moving the film cassette from one image position to another. When utilized in conjunction with a system that takes a series of rapidly sequenced exposures the prior art oscillation approaches of reducing the appearance of grid lines exhibit the same problems previously discussed. In addition to the previously mentioned drawbacks, the Stava invention does not provide an apparatus for performing a series of rapidly sequenced exposures on separate subsections of the image producing element while ensuring that the grid is not stationary during an appreciable portion of each exposure.
It is the object of this invention to overcome these problems and others by providing a new and improved apparatus and method to minimize the formation of grid lines for a single exposure of a sheet of film or a sequential series of exposures of separate subsections of a sheet of film.